Implement files.exists in terms of file.statOrNull.
Implement files.exists in terms of file.statOrNull.

The hitherto-underlying fs.exists function has been deprecated in Node v4.

Part of #6921.

jdivy/meteor,4commerce-technologies-AG/meteor,chasertech/meteor,Hansoft/meteor,chasertech/meteor,mjmasn/meteor,mjmasn/meteor,4commerce-technologies-AG/meteor,Hansoft/meteor,4commerce-technologies-AG/meteor,mjmasn/meteor,4commerce-technologies-AG/meteor,Hansoft/meteor,jdivy/meteor,Hansoft/meteor,4commerce-technologies-AG/meteor,chasertech/meteor,mjmasn/meteor,4commerce-technologies-AG/meteor,Hansoft/meteor,mjmasn/meteor,chasertech/meteor,jdivy/meteor,jdivy/meteor,mjmasn/meteor,mjmasn/meteor,jdivy/meteor,4commerce-technologies-AG/meteor,chasertech/meteor,jdivy/meteor,jdivy/meteor,Hansoft/meteor,chasertech/meteor,chasertech/meteor,Hansoft/meteor